The practical implementation of Smart Material and Structure technology will be greatly facilitated by the present invention as it addresses a number of critical challenges facing this new field. We shall use the term Smart Material (or Smart Structure) to mean a material or structure having a resident fiber optic sensing system {optical fiber sensor will be used in a generic manner to mean any form of optical guided wave sensor} as part of the material or structure. This structurally integrated sensing system can be adhered to the surface of the material (or structure) or embedded within the material (or structure). The former situation is more often then not likely to arise for a metal, while the latter case is especially appropriate for: advanced composite materials, concrete, and in certain situations, cast metals.
These smart materials (or structures) could greatly improve the safety and economics of many industrial sectors: from aircraft to bridges, from space structures to submarines, from ships to pressure vessels. This structurally integrated sensing technology could also help protect the environment, first by warning of possible leaks or impending failure of containment vessels, and second by greatly reducing the need for inspections that require paint stripping of structures {like aircraft}. Structurally integrated sensing systems would also constitute a necessary component of any Smart Adaptive Structure controlled by an integrated actuation system.
The OSSI invention would constitute the heart of such structurally integrated sensing systems and represents a significant advance toward the practical implementation of Smart Material and Structure technology because it makes possible:
(i) A low cost, compact, robust, unobtrusive and nonperturbative interface which is capable of transmitting the sensing information from the structure through one of several user-friendly modes, including noncontact methods. To understand the significance of this advance it is necessary to appreciate that current approaches are based on conducting the raw optical signals from the optical fiber sensors out of the structure through a mechanical interconnect. This represents a major challenge for practical structures, such as wings of aircraft {which are subject to regular inspection, maintenance and repair, sometimes in harsh environments} because all fiber optic sensors deemed to be most appropriate for use with Smart Structures are based on single mode optical fibers and so very accurate and consistent alignment (of the order of a .mu.m) will be required. By converting the raw optical signals to processed and multiplexed electronic signals within the OSSI we overcome these problems and can interconnect to the structure with either a single robust electrical cable {or single multimode optical fiber} or a noncontact free-space optical {or radio frequency} propagation technique. This latter technique becomes very important when the structure is inaccessible, isolated, or not convenient to require wire or optical fiber connections, and can be thought of as an "optical synapse." PA1 (ii) A "sensing cell architecture" that offers great flexibility in design and configuration of the sensing system. This flexibility will be required if fiber optic sensing systems are to be structurally integrated within complex structural components, especially those fabricated from multilaminate advanced composite materials. PA1 (iii) A low cost implementation of this technology when used in large numbers due to the possibility of automated manufacture of optoelectronic packages. PA1 (iv) A high degree of damage tolerance for the sensing system that would permit graceful degradation of the sensing system in the event of damage to the structure. PA1 (v) An adaptive neural network architecture in which each OSSI serves as a node and is connected to one or more other OSSI's of the same structure to form a self learning system that might also be self compensating or correcting.
An important embodiment of this OSSI invention is the development of several active wavelength demodulation systems for laser sensors that permit a large number of such demodulation systems to be built on a single monolithic optoelectronic substrate that would also be capable of undertaking both electronic signal processing and multiplexing. This leads to an optoelectronic interface is small and rugged enough to be integrated with almost any kind of structural component.
The present invention would find use in almost any kind of Smart Material (or structure) and would permit low cost implementation of this technology when used in large numbers due to the combination of several key functions onto a single optoelectronic substrate to form an interface that is compact and rugged enough to be integrated with the structure and the possibility of automated manufacture. These functions include: receiving external energy to power the system, interrogation of the sensing cell array of laser sensors, demodulation of the laser sensor signals,electronic processing and multiplexing of the sensing data into a single output channel, and transmission of this sensing data from the structure by means of a user friendly interface. The OSSI could be designed to use one or more output {communication} modes so that one system could be used in a variety of interconnect situations. These communication modes include: a single electrical (or optical) cable and a simple mechanical interconnect, or remotely through a noncontact approach based on free-space optical propagation. In the case of more isolated, or very large structures, like bridges, it may even be desirable to use radiowaves and this is also feasible with the OSSI. The nature of the input port of the OSSI would to some extent depend upon the output mode, the physical site and use of the structure. If the output data is transmitted as a free-space beam of optical radiation and avoidance of electrical cables is desirable, it would make sense for the input port to be designed to receive optical energy to power the OSSI.
This interconnect freedom and the combination of functions undertaken by the OSSI would permit a "sensing cell architecture" that would provide considerable flexibility in the sensing configuration, allowing a comprehensive sensing system to be integrated into structures of complex multilaminate shape with differing sensor density and configuration requirements in different regions of the structure. Another important feature of this OSSI invention is that it makes possible the development of a structurally integrated information processing network for handling the enormous flow of complex signals that would arise from a large number of sensing cells. This processing network could be used to reduce this data to a more meaningful flow of highly relevant information. In this architecture each OSSI might serve as a neuron in a neural network, each with its attendant set of sensing optical nerves. This might allow considerable signal reduction and interpretation within the adaptive network. An information network based on OSSI's need not restrict its use to transmitting and processing of sensing information collected by the attendant sensing system, it could also serve as a communication network for other information. For example, in a future fly-by-light aircraft, a structurally integrated OSSI based optical sensing network might double as a communication system for the optical flight control signals.